Digital printers or copier machines are known which operate according to the electrophotographic process, in which a latent electrostatic image is developed by charged toner particles. These particles are transferred onto an image receiving substrate, hereinafter referred to simply as “substrate”. Afterwards, the developed image that has been transferred onto the substrate is fixed by the toner particles being heated and fused. In order to fuse the toner particles, contacting processes are often used in which the toner particles are brought into contact with suitable devices, for example, hot rollers or cylinders. It is disadvantageous that it is usually necessary to use silicone oil as a separating agent that should prevent an adhesion of the fused toner onto the heating device. In addition, the design, the maintenance and the operating costs of these heating devices that operate by contact are expensive and thus cost-intensive. Furthermore, the defect rate caused by the contacting heating devices is relatively high. Nevertheless, process methods of this type continue to be popular and make possible a fixing of the toner that acts by pressure impingement into the structure of the substrate.
In order to fix the toner that has been transferred onto the paper, for example, heating devices and processes are also known that operate in a contactless manner, in which for example, the toner particles are fused using heat radiation and/or microwave radiation or with hot air.
In the contacting and non-contacting fusing processes, toner is used, for example, that has a glass transformation temperature (TG) in a range from 45° C. to 75° C. The glass transformation temperature, at which the toner—starting from the solid state—begins to soften, can be influenced by the choice of raw materials and by the addition of certain additives to the toner. In a fixing device for the toner, which has at least one heating device, both the toner and the substrate itself are heated up. In order to be able to ensure a good fixing of the toner onto the substrate, the surface temperature of the substrate must be in the range of the glass transformation temperature of the toner or above it. The toner easily reaches and/or exceeds the glass transformation temperature (TG) in the area of the heating device.
Printers and copier machines are known in which the substrate is printed double-sided, whereby for the printing of the front side and the rear side, either one and the same image creation and transfer device and heating device, or a separate image creation and transfer device and a separate heating device are each used. In order to fix the toner image, the substrate is often guided past the at least one image creation and transfer device and the allocated heating device using a conveyor belt on which the substrate lies. In the process, at first a first toner image is transferred onto a first side of the substrate and fixed onto it. Then, a second toner image is transferred and fixed onto the second side of the substrate. During the fusing of the second toner image, the first side of the substrate with the first toner image that is located on it and has already been fixed onto it, thus rests on the conveyor belt. It is disadvantageous in this process that during the fusing of the second toner image, the first toner image can heat up so much that is becomes soft and tends to adhere to the conveyor belt. This can lead to several undesired effects: Due to the adhesion, the substrate can become jammed when the substrate is passed from the conveyor belt to a subsequent part of the machine. Furthermore, the appearance of the toner image can change in the areas in which it is adhered to the conveyor belt. This leads to problems with the image quality; for example, the toner image has a non-uniform gloss. These undesired effects are amplified by pressure impingement with a hot roller when the toner is fixed by contact.
Conveyor belts are known which are constructed as a suction belt, i.e. they have a number of openings through them that are impinged by a vacuum and on which the substrate can be suctioned and in this way held. The known suction belt has the same disadvantages as a conveyor belt with a solid supporting surface.